Cell (Mobile) Phones in Japan
Japan is called "Cell Phone Galapagos Islands" after all new technologies Japanese companies have developed.
Many foreign cell phones do not work in Japan because of the difference of network although many of new models work. And Japanese cell phone users can't insert their SIM card in foreign countries.
To prevent any annoyance, I recommend the following.
Calling to Your Home Country
Skype is the best way to call your family, friends etc at home.
See Internet Access and Electrical Equipments to use your laptop and Skype in Japan.
Internet access is available in most hotels, Starbucks and some public places in Tokyo.
Skype is a FREE Internet call service provided for most countries in the world. Installing and calling on Skype is completely free and simple to use.
In case you haven't got it, let's learn it.
http://www.skype.com/en/ (You can select your own language.)
Calling Inside Japan
Rental Cell Phone and SIM Card is the best way.
There are some reasons.
First, it is difficult for many people to understand professional knowledge about networking and cell phone models. If you thought your cell phone would work in Japan and it doesn't actually, you will worry about it. You can prevent that trouble with a rental phone.
Second, even if your foreign cell phone works inside Japan, it is possible that calling costs much.
At last, you cannot call Emergency Numbers and most telephone numbers via Skype.
A rental cell phone solves these problems.
(Tip: Want to know Emergency Numbers?)
Rental cell phone and SIM card is available at the airports.
NTT Docomo, Softbank and au by KDDI are Japanese leading cell phone companies and they run many shops in the airports. They offer convenient services and you need only your passport and credit card to get a rental cell phone. You can return it at the airport or mail it to their office. Please ask them when you get one.
The fee depends on the company and plan, so I show a model here.
Daily Fee (200 - 500 yen for a cell phone, 1000 - 1500 yen for a smart (tablet) phone) + Calling Fee (40 - 100 yen per minute).
Do you definitely want to know whether your cell phone works or not in Japan?
Your cell phone need to be compatible with Japanese cell phone networks.
They include 3G UMTS 2100 MHz, 3G CDMA2000 800 MHz, and LTE band 1. Many of new 3G and 4G phones are compatible with at least one of them.
GSM-Only phones do not work because there are not GSM Network in Japan.
The best way is to ask your cell phone company with this information.